1. Field of the Invention
The present invention relates to an optically parametric oscillator, and more particularly to an optically parametric oscillator wherein wavelengths of signal light and idler light are allowed to vary by changing the wavelength of excitation light.
2. Description of the Related Art
In general, an optically parametric oscillator of an excitation wavelength tunable type wherein a resonator is composed of an output coupling mirror having a prescribed transmittivity and a total reflection mirror, a nonlinear crystal is disposed in the resonator, excitation light is inputted into the resonator to excite the nonlinear crystal, whereby signal light and idler light both of which have wavelengths in response to a wavelength of the excitation light, respectively, are outputted has been known.
In such an optically parametric oscillator as described above, a position for providing the nonlinear crystal has been fixed and when a wavelength of the excitation light is varied, the signal light and the idler light having a desired wavelength, respectively, are obtained.
In this respect, as an excitation light source for generating excitation light having an arbitrary wavelength, wavelength tunable laser is usually employed. As the wavelength tunable laser, a solid laser wherein a crystal such as Ti:Al.sub.2 O.sub.3 (titanium sapphire) and the like is used as the laser medium, and a liquid laser wherein a dye solution or the like is used as the laser medium have been widely employed.
Heretofore, as a manner for selecting wavelength for the sake of producing laser oscillation, there is, for example, such an arrangement that a diffraction grating, birefringent plate or the like is disposed in a laser resonator containing a wavelength tunable laser medium, such a diffraction grating, birefringent plate or the like is mechanically rotated, whereby only the outgoing light having a desired wavelength is taken out from among the outgoing light rays outputted from the wavelength tunable laser, the outgoing light thus taken out reflects with respect to the wavelength tunable laser to amplify the same thereby producing laser oscillation, so that only the laser beam having a desired wavelength is outputted from the laser resonator.
However, such a conventional manner for selecting wavelength as described above involves such a problem that since a diffraction grating, birefringent plate or the like is mechanically rotated in this manner, it is difficult to speed up a wavelength tuning speed of the laser beam outputted as excitation light, so that both wavelengths of signal light and idler light cannot be varied at high speed.